Storage tanks for oil and other liquid petrochemical products are typically provided with floating roofs that float atop the liquid product and seal the upper portions of the tanks to prevent the escape of harmful vapors off the surface of the contained liquid. Additional seals are provided between the outer periphery of each floating roof and the inner wall of the tank.
Such floating roofs are usually circular in shape, light weight and buoyant, typically may have a diameter of fifty feet or more, and are relatively thin and relatively weak.
A serious potential problem or danger with such floating roofs is that under rainstorm conditions water may accumulate on top of the roof and cause it to sink or break. Under these circumstances vapor from the liquid below the roof could escape into the atmosphere or into the space in the tank but above the roof, and produce undesirable and/or dangerous conditions.
In the prior art there are numerous drainage systems for such accumulated water atop a floating roof, and also emergency drainage systems which are intended to drain automatically whenever the accumulated water reaches a pre-determined dangerous level.
Disadvantages of prior art drainage systems include unreliable valves which stick in a closed or open position, and valves which are expensive or difficult to maintain, repair or install. With such prior art valves the sealing element may not reliably respond to a flow of accumulated water on the roof, or is too slow in reaction time to open the drain.
Prior art patent U.S. Pat. No. 5,758,792 discloses, for example, in FIG. 30 a floating ball check valve 220 on a floating roof where the plane of the closure is at the top surface of the roofs top deck sheeting 24. With this arrangement the ball 236 will not rise and open the valve until a substantial amount of water has accumulated upon the roof. During drainage, another feature of this device is that the ball will float down and close while a certain quantity of water still remains on the roof. Thus, this valve's main purpose is to block upward flow of vapors after the main drainage of water has occurred; it does not address the problem of massive accumulation of water before the valve opens or the problem of water remaining on the roof after the valve has closed.